Diapause induction and clock mechanism in the cabbage butterfly Pieris melete Ménétriés

Photoperiodic control of diapause induction was systematically investigated in the cabbage butterfly, Pieris melete, which enters summer and winter diapause as a pupa. Summer and winter diapause are induced principally by short and long scotophases, respectively; the intermediate scotophases (11–12 h) permit pupae to develop without diapause. Photoperiodic responses under 24-h light–dark cycles at 16.9, 18, 20 and 22 °C showed that the hibernation response was temperature compensated, whereas aestivation response was strongly temperature-dependent. The incidence of diapause for both aestivation and hibernation showed a decline at the ultra-short and ultra-long scotophases. Experiments using non-24-h light–dark cycles showed that the length of the scotophase played an essential role in the determination of diapause. The highest photosensitivity differed under hibernation and aestivation conditions. With a 3 × LD 12:12 interruption, a maximal inhibition of aestivation occurred in the L3/2 stage, and of hibernation it occurred in the L4/0 stage. A long-night of LD 10:14 induced hibernation diapause but inhibited aestivation diapause and, conversely, a short-night of LD 14:10 inhibited hibernation diapause but induced aestivation diapause. With a 1-h light pulse at LD 11:13, a maximal inhibition of hibernation occurred 3 h before lights-on (late scotophase), whereas, with a 1-h light pulse at LD 12.5:11.5, a maximal induction of aestivation occurred 2–3 h after the onset of darkness (early scotophase). Nanda-Hamner and Bünsow experiments failed to reveal the involvement of a circadian system, suggesting that the photoperiodic time measurement for diapause induction in this butterfly resembles an hourglass-like timer or a damped circadian oscillator.